Many tumbling media mills (TMMs) are in use with such grinding media as balls, slugs, rods, tubes and the like. Although high in power consumption, TMMs offer a number of advantages. As compared to hammermills, granulators, jaw crushers, multi-roll mills and other grinding and crushing equipment, TMMs more readily handle a wide range of soft to very hard materials and grind them to very fine products. TMMs are also rugged in construction, simple to operate, and withstand the ravages of abrasive materials and uncrushable objects. This is why TMMs are popular for applications in which the feed stream contains uncrushable tramp materials or is very abrasive, and in which simple operation is desired more than low specific power consumption.
A typical feeding arrangement for a TMM is a weigh conveyor controlled by a feed-back loop from sensors selected and located to measure whether the mill is being over- or under-fed. Such sensors as microphones to "listen" to the sound of the mill and/or current measuring equipment to monitor the load on a product conveyor at the mill solids discharge outlet have been used.
Grinding efficiency has been a major goal in the design and operation of other types of grinding equipment. However, in a TMM, because the weight of the tumbling bed of media in the grinding chamber greatly overshadows the weight of material undergoing grinding at any given time in normal operation, TMMs typically operate within a narrow range of efficiency, leaving little apparent opportunity to benefit from major efforts at improving efficiency.
Intelligent and knowledgeable operators can usually tell if a TMM is significantly over- or under-fed by listening at specific points on the mill shell. However, efficiency of mill operation cannot be determined easily or in "real time" measurements. Data must be collected on the feed material size and hardness, product size distribution, presence of moisture, power measurements, and other circuit parameters. These facts must be evaluated together to give a resultant system efficiency.
Such efforts as have been made to increase efficiency through improved TMM apparatus and operating methods have mainly focused on providing a strong current of air through the grinding chamber to sweep as much as possible of the finest material out of the bed as soon as possible. Also, heating the air is of assistance in drying the bed, its contents and the recovered fine particles when the feed is moist.
There is a need for improvements in the design of TMM feeding and control systems to provide for removal of product size particles from the feed stream, particularly particles of substantial moisture content, before the material enters the mill. Also, there is a need to control the feed stream into the TMM to maximize the ability of the TMM to crush and grind more efficiently. The present invention fulfills these needs and others, as will become clear from the description of the invention and of various embodiments thereof which follows.